A treatise from a former JW. Wish I could say I did all the digging on this info, but this all straight out of their publications:
Back in the days of Charles Taze Russell, the Watchtower Society correctly taught that Abaddon/Apollyon from Revelation chapter 9 was Satan or a demon (because the Bible describes him as a fallen angel of the Abyss who is in control of all of the demon/locusts):
“The Finished Mystery” (1917):
"[And] they [had a] HAVE THEIR king [over them]. — The same king as exercises general rulership over all the ecclesiastical affairs of this present evil world. [Which is] the angel of the [bottomless pit] ABYSS. — "The prince of the power of the air." — Eph. 2:2. Whose name in the Hebrew tongue is Abaddon. — And he is "a bad one," sure enough. — 2 Cor. 4:4. But in the Greek tongue hath his name Apollyon. — That is, Destroyer. But in plain English his name is Satan, the Devil. As a means for preventing people from seeing the Truth, Methodism has been as efficient as any of the five systems previously described, or any that follow. All have been sadly misled and deceived by our great and wily Adversary; "Taken captive by him at his will." — 2 Tim. 2:26."
However, in modern times (starting at least in the 1950's), Jehovah's Witnesses teach that the Fallen Angel of the Abyss who controls the army of demon/locusts is JESUS CHRIST, and that the demon/locusts ARE JEHOVAH'S ANOINTED ONES:
The Watchtower, May 1st, 1992, Page 11:
10 This is a campaign extraordinaire, one that will be memorialized for all time. Wave upon wave of insects, most prominently locusts, devastate the land. What does this mean? Revelation 9:1-12 also speaks of a plague of locusts, sent forth by Jehovah under “a king, the angel of the abyss,” who is none other than Christ Jesus. His names Abaddon (Hebrew) and Apollyon (Greek) mean “Destruction” and “Destroyer.” These locusts picture the anointed remnant of Christians who, now in the Lord’s day, go forth to devastate Christendom’s pastures by completely exposing false religion and proclaiming Jehovah’s vengeance upon it.
"Revelation--Its Grand Climax At Hand!" Book (1988), Page 148:
Rather than announce the incoming Kingdom of God, Christendom’s clergy have chosen to remain with Satan’s world. They want no part with the locust band and their King, concerning whom John now observes: “They have over them a king, the angel of the abyss. In Hebrew his name is Abaddon [meaning “Destruction”], but in Greek he has the name Apollyon [meaning “Destroyer”].” (Revelation 9:11) As “angel of the abyss” and “Destroyer,” Jesus had truly released a plaguing woe on Christendom. But more is to follow!
This is supported by the use of Hebrew names in other visions; Jesus is given the Hebrew name “Abaddon” (meaning “Destruction”) and executes judgment at the place “called in Hebrew Har–Magedon.”—Revelation 9:11; 16:16.
The Watchtower, December 1st, 1969, Page 719:
With a loud trumpet peal the fifth angel announced the loosing of the abyssed “witnesses” and their work that followed. Was this to be a “woe” to “those dwelling on the earth”? With prophetic vision the apostle John watched to see. Look! A “star” had fallen, not into the sea or into the fresh waters, but to the earth. But its fall was not disastrous to it. Rather, this “star” comes as a Freedom-giver, a Liberator of others, for “the key of the pit of the abyss was given him.” He is also a King, for he is “the angel of the abyss” and is the King of those whom he releases from the abyss; in harmony with which fact those whom he releases wear what, to John, “seemed to be crowns like gold.” This symbolic “star” also has a name for this occasion. In Greek its name is Apollyon, which means Destroyer. In Hebrew it is Abaddon, which means Destruction. All these features about this symbolic “star” reveal him to be the glorified Jesus Christ.
The Watchtower, December 1st, 1961, Page 719:
The symbolic locusts must follow their King, Jesus Christ. John says of them: “They have over them a king, the angel of the abyss. In Hebrew his name is Abad´don, but in Greek he has the name Apollyon.” The resurrected heavenly Jesus Christ is the “angel of the abyss,” for he holds the “keys of death and of Ha´des.”
The Watchtower, March 1st 1951, Page 149:
These “locusts” are the army of Jehovah’s witnesses, and as to their effect upon Christendom’s “rich man” class Revelation 9:5-7, 10, 11 says: “It was granted the locusts, not to kill them, but that these should be tormented five months, and the torment upon them was as torment by a scorpion when it strikes a man. And in those days the men will seek death but will by no means find it, and they will desire to die but death keeps fleeing from them. And the likenesses of the locusts resembled horses prepared for battle, . . . Also they have tails and stings like scorpions; and in their tails is their authority to hurt the men five months. They have over them a king, the angel of the abyss. In Hebrew his name is A·bad´don [Destruction], but in Greek he has the name A·pol´lyon [Destroyer].” (NW) Not surprising that the effort is made throughout Christendom to destroy Jehovah’s witnesses.